Diverter heads

ABSTRACT

A monitoring system for a rotating diverter head for use in the drilling of oil, gas and geothermal wells. The system includes a pressure sensor which is mounted beside the stripper rubber which contacts the drill pipe. An increase in the pressure monitored provides early warning of degradation or imminent failure of the seal. Apparatus and method are also described for the system with a head having dual stripper rubbers. Additionally a support for locating a housing of the head is provided so that the stripper rubbers can be replaced on-site.

The present invention relates to equipment used in the drilling of oil, gas and geothermal wells and in particular, though not exclusively, to a diverter head which includes a monitoring system to provide early warning of degradation or imminent failure of a seal within the head.

In drilling a well, a drilling tool or “drill bit” is rotated under an axial load within a bore hole. The drill bit is attached to the bottom of a string of threadably connected tubulars or “drill pipe” located in the bore hole. The drill pipe is rotated at the surface of the well by an applied torque which is transferred by the drill pipe to the drill bit. As the bore hole is drilled, the hole bored by the drill bit is substantially greater than the diameter of the drill pipe. To assist in lubricating the drill bit, drilling fluid or gas is pumped down the drill pipe. The fluid jets out of the drill bit, flowing back up to the surface through the annulus between the wall of the bore hole and the drill pipe.

The density of the drilling fluid is adjusted such that the pressure head produced by the weight of the column of drilling fluid is slightly more or less than the pressure of the oil or gas encountered in the geological formations being drilled through. If the pressure head of the column of drilling fluid is greater than the pressure of the oil or gas, the top of the well can be open to atmosphere. It is often advantageous to allow the pressure head of the drilling fluid to be slightly less than the pressure of the oil or gas encountered in the formation. In this case, known as “underbalanced drilling”, the annulus around the drill pipe needs to be sealed and the drilling fluid returning under pressure up the annulus must be diverted to a recirculating unit for pumping back down the well.

Rotating diverter heads provide a means of sealing off the annulus around the drill pipe as the drill pipe rotates and translates axially down the well while including a side outlet through which the return drilling fluid is diverted. Such rotating diverter heads may also be referred to as rotating blow out preventers or drilling heads. These units generally comprise a stationary housing or bowl including a side outlet for connection to a fluid return line and an inlet flange for locating the unit on a blow out preventer or other drilling stack at the surface of the well bore. Within the bowl, opposite the inlet flange, is arranged a rotatable assembly such as anti-friction bearings which allow the drill pipe, located through the head, to rotate and slide. The assembly includes a seal onto the drill pipe which is typically a strip of rubber. The stripper rubber is exposed from one side by the well pressure and this, combined with the extreme working conditions, means that the stripper rubber wears easily and as a result is subject to failure at regular intervals. On failure, hazardous well fluids, steam or hot water vapour can escape causing injury to personnel and result in unwanted downtime while the system is brought under control and the stripper rubber replaced.

Prior art rotatable diverter heads have concentrated on improvements to the sealing means, in particular by providing dual stripper arrangements to improve the seal onto the drill pipe, while increasing the ease to which the stripper rubbers can be replaced to reduce the downtime following seal failure. For example, U.S. Pat. No. 5,662,181 describes a rotating blow out preventer wherein upper and lower stripper rubbers are used which are mounted in a single releasable assembly to minimise maintenance downtime. Additionally chilled water or antifreeze is circulated around the upper stripper rubber and bearings in an attempt to lengthen seal and bearing life by at least partially offsetting well pressure. Such cooling is of limited advantage to the lower stripper rubber and the coolant system increases the complexity of mounting the head. Thus the lower stripper rubber is still prone to failure through wear and exposure to the well pressure.

U.S. Pat. No. 5,213,158 provides a drilling head with dual rotating stripper rubbers designed for high pressure drilling operations to ensure sealing under the extreme conditions of high flow or high pressure wells. The stripper rubbers have the same diameter but are manufactured from different materials, the lower stripper rubber being a rigid, abrasive resistant material to divert the flow from the well while the upper stripper rubber is of a softer sealing material that closely conforms to the outer diameter of the drill string thereby preventing the flow of fluids through the drilling head. In this arrangement, pressure can seep to the upper stripper rubber and cause failure thereof. Further, there is an additional disadvantage in that supplies of two different rubbers must be available and these rubbers must be fitted in the correct order within the head.

It is an object of the present invention to provide a rotating diverter head in which stripper rubber failure is prevented.

It is a further object of the present invention to provide a method of preventing stripper rubber failure in a rotating diverter head.

According to a first aspect of the present invention there is provided a rotating diverter head comprising:

-   -   a bowl member having a first bore aligned on a central axis         therethrough and a second bore located substantially transverse         of the central axis;     -   a housing located substantially within the bowl member including         rotational means to rotate the housing relative to the bowl         member;     -   first sealing means to sealably engage upon a drill pipe when         the drill pipe is inserted through the first bore, and     -   pressure detecting means to determine pressure at a side of the         first sealing means.

As a lower side of the sealing means will be exposed to the well pressure, measuring pressure on the upper or opposite side of the sealing means will indicate if well pressure is passing the sealing means. A measured increase in pressure will show that the sealing means is failing. In this way the sealing means can be replaced before failure occurs.

Preferably the first sealing means is a stripper rubber as is known in the art. Preferably the first sealing means is located on the housing such that the sealing means will rotate with the housing relative to the bowl member.

Preferably also the head comprises a second sealing means to sealably engage upon the drill pipe. The second sealing means may be located above the first sealing means so that the drill pipe engages the second sealing means prior to the first sealing means when inserted into the first bore. Preferably also the second sealing means is a stripper rubber as is known in the art.

Preferably the first and second sealing means are substantially the same. In this way the system only requires a single supply of stripper rubbers which prevents incorrect assembly.

Preferably the pressure detecting means is located between the first and second sealing means. In this way early warning of failure of the first or lower sealing means is provided.

Preferably the detecting means comprises a pressure sensor. More preferably the sensor measures the pressure differential between the sealing means. Preferably the pressure sensor is located at a port on the housing above the first sealing means. For a dual sealing means system the port may be located between the sealing means, preferably adjacent the second sealing means. Advantageously a valve is located in the port. Advantageously also a quick release coupling is located at the port. In this way the pressure sensor can be attached to the port when a measurement is required and thereafter removed so that the housing can rotate freely. In an alternative embodiment the pressure sensor includes a transmitter. Such an embodiment allows the housing to rotate while measurements are made. It also allows real-time measurement of pressure to be made.

Preferably also the detecting means comprises a warning means. The warning means may be an audible and/or visual indicator to alert a user when the pressure detected has reached a predetermined level indicating that the lower stripper rubber should be changed.

The diverter head may be supplied with a housing support. The housing support may comprise a stand and a rotatably mounted holder adapted to support the housing when inserted therein. In this way the housing can be supported and rotated for the stripper rubber(s) to be replaced on-site without the need to return the housing or the head back to workshop for the worn stripper rubber(s) to be replaced. This reduces the down time for stripper rubber replacement.

According to a second aspect of the present invention there is provided a method of preventing stripper rubber failure in a rotating diverter head, the method comprising the steps:

-   (a) providing a port in the head above the stripper rubber; -   (b) connecting a pressure sensor to the port; -   (c) measuring the pressure at the port; -   (d) recording consecutive pressure measurements over time intervals; -   (e) noting when the measurements show an increase in pressure     greater than a predetermined pressure value; and -   (f) replacing the stripper rubber.

The pressure value is chosen to be less than the operating pressure in the well and thus the measured increase to the pressure value will indicate that the stripper rubber is wearing and likely to fail shortly. A controlled shut-down can then take place prior to failure of the stripper rubber. Additionally the value can be set so that the maximum life of a stripper rubber is obtained before replacement.

As the port is preferably located on the housing, the method preferably includes the step of holding the housing stationary and locating the sensor in the port for each measurement. This prevents the pressure sensor from interfering with the rotational movement of the housing during use of the head.

In an alternative embodiment, the pressure sensor may be permanently located on the housing and pressure measurements are relayed via a transmitter to a receiver located distant from the head, wherein the measurements are recorded. Such measurements may be made in real-time.

In a head with dual stripper rubbers, the port is preferably located below the upper stripper rubber to measure the pressure differential between the stripper rubbers.

The method may further include the step of bleeding off residual pressure once the pressure measurement has been taken.

The method may further include the step of providing an audible and/or visual indicator when the predetermined pressure value has been reached. This will alert a user to change the stripper rubber.

The method may further include the steps of:

-   (a) removing a portion of the head including the stripper rubber; -   (b) locating the portion on a rotatable support positioned adjacent     a well head; -   (c) replacing the stripper rubber.

In this way the stripper rubber can be replaced on-site. Preferably the portion is the housing of the head.

Example embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings of which:

FIG. 1 is a cross sectional view taken vertically through an a rotating diverter head according to an embodiment of the present invention; and

FIG. 2 is a schematic illustration of a pressure monitoring system for a rotating diverter head according to a further embodiment of the present invention.

Reference is initially made to FIG. 1 illustrating a rotating diverter head, generally indicated by reference numeral 10, in accordance with an embodiment of the present invention.

Head 10 includes a bowl 11 which is generally a cylindrical body, a rotating spindle 12, an inlet flange 14 and a side outlet 16. Spindle 12 forms a housing which rotates in anti-friction bearings 18. Spindle 12 also includes a seal 20 which sealably engages a drill pipe (not shown) located through a central bore 22 of the head 10. Well fluids in the form of drilling fluids from the well below are thus prevented from travelling up through the head 10 and are diverted out of the head 10, through the side outlet 16.

In the embodiments shown there is a second upper seal 24 also located on the housing or spindle 12. Seals 20, 24 are what are termed in the art “stripper rubbers”. These comprise a moulded resilient material having a through hole 26, 28 respectively, and a flange 30 where the seal 20, 24 is connected to the housing 12. A nominal diameter of the through hole 26, 28 is somewhat smaller than the diameter of the drill pipe (not shown), such that the inner surface 32, 34 of the through hole 26, 28 sealably engages on an outer diameter of the drill pipe (not shown).

The arrangement of seals 20, 24, the housing or spindle 12 and the bowl 11 is as would be found typically on a diverter head. Thus, the embodiment shown in FIG. 1 is one example of a diverter head and it will be appreciated that the invention could be worked on any diverter head arrangement having a seal on to drill pipe, inserted through the seal. Additionally, the seal 20 could be located on the bowl 11, and thus would not rotate with the spindle 12 in use. However, it is recognised that it is advantageous to have the seals 20, 24 located on the spindle 12, so that they can rotate with the drill pipe to reduce wear upon these components.

Located on the spindle 12, above the lower seal 20, and below the upper seal 24, is a port 36. Port 36 is located through the housing wall 38 and provides access to a portion 50 of the bore 22 between the seals 20 and 24. Port 36 has located thereon a valve 40. Valve 40 is typically a one-way valve, which can be arranged to allow fluid pressure from the bore 22 to escape through the port 36. Also located at the port 36 is a quick release coupling 42. Coupling 42 provides for attachment of a pressure sensor 44 or for a hose (not shown).

In the embodiment disclosed, pressure sensor 44 is mounted upon the spindle 12 and includes power supply 46 and a transmitter 48. In this way the pressure exiting in the portion 50 between the seals 20 and 24 is recorded by the pressure sensor 44. The pressure measurement is relayed by the transmitter 48 to a remote site where the measurement is recorded.

In use the head 10 is lifted, typically by lugs (not shown), to be located upon a blow out preventer stack (not shown). Flange 14 has a base 54 compatible with the top flange of the blow out preventer stack, and the two are linked via screws. This is known in the art. The dimensions of the base 54 of the flange 14 are determined by an international standard to ensure proper mating with other flanges of the same size and pressure rating. Once positioned, the flange 14 is fixed in relation to the blow out preventer stack. A seal groove 52 on the bottom face 56 of the base 54 provides for an o-ring to be inserted to prevent the egress of fluid from the head 10 between the base 54 and the blow out preventer stack.

In the embodiment shown, the inlet flange 14 and the bowl 11 are separate components to provide an advantageous feature of allowing the bowl 11 to be rotated upon the flange 14 once positioned, so that the side outlet 16 can be rotated with respect to the central bore 22 to aid alignment of the side outlet 16 with return flow lines. The coupling between the bowl 11 and the inlet flange 14 will be described hereinafter.

With the head 10 mounted upon the blow out preventer stack, a drill pipe can be inserted through the bore 22. The drill pipe will contact the upper seal 24 before reaching the lower seal 20, wherein it is forced through this lower seal and through the central bore 22 into the well below the blow out preventer stack. An outer surface of the drill pipe contacts both the seals 20, 24 to prevent fluid passing from the well into the portion 50 and also through the head 10. Fluid is thus diverted from the well through the side outlet 16 to be processed and returned to the well via the drill pipe. This is as known in the art.

In use the drill pipe is rotated and reciprocated through the head 10. The seals 20, 24 will rotate with the drill pipe by virtue of the bearings 18, and thus the spindle 12 will rotate with the drill pipe also. This rotation will be relative to the stationary bowl 11 and inlet flange 14. As will be appreciated, rotation and reciprocation of the drill pipe will cause relative movement between the outer surface of the drill pipe and the inner surface 32, 34 of the seals 20,24. Additionally, the well fluid pressure will act upon the lower seal 20. This excess pressure, together with the abrasive movement of the drill pipe will cause the lower seal 20 to wear on the inner surface 32. Wear on the inner surface 32 will increase the size of the through hole 26, and as a result well fluids will be able to leak between the drill pipe and the seal 20 into the portion 50. Repeated use of the drill pipe will cause repeated wear upon the inner surface 32, and eventually the seal provided by the stripper rubber 20 against the drill pipe will degrade and fail. When this occurs, well fluids can enter the portion 50 and past the upper seal 24 as there will be similar wear against the inner surface 34 of the upper seal 24, and as a result well fluids can exit the head 10 at the upper end 58. In prior art diverter heads, when this occurred and fluids escaped, this provided the only indication that the seals required replacement. The entire production of the well would be stopped. The drill pipe would be removed from the well and the blow out preventer stack. The head 10 would then be removed and taken onshore where it was disassembled and the seals 20 and 24 replaced. The head is then returned to the well head and located on the blow out preventer stack again. During typical drilling of a well, the stripper rubbers 20, 24 will need to be replaced a significant number of times. Each replacement provides an unwanted downtime on the well, while allowing the seals to fail before replacement is a safety risk, as fluid is allowed to escape from the head 58.

In the present invention, during the rotation and reciprocation of the drill pipe through the seals 20, 24, the pressure differential in the portion 50 is measured via the pressure sensor 44. The pressure sensor 44 can measure the rise in pressure which will occur be experienced as the inner surface 32 of the lower seal 20 begins to fail. Thus, any escape or leakage of well fluids from the well into the portion 50 will be detected by the pressure sensor 44. Such pressure measurements can be made at regular intervals, or can alternatively be done in continuous real-time by the pressure sensor 44. In the embodiment shown, the pressure sensor 44 can operate in real-time. In an alternative embodiment, the port 36, together with the valve 40 and quick release coupling 42 are located on the spindle 12. Only at the times of pressure measurements will a pressure sensor be located onto the quick release coupling for an instant pressure measurement to be made. Such measurements will be made at regular intervals, and will require the drill pipe to be stationary, such that the spindle will be stationary when the measurement is taken. For the embodiment shown in FIG. 1 wherein the pressure sensor is connected to the spindle 12, there is no need to stop the drill pipe and spindle from rotation, as measurements can be made during rotation of the spindle 12. In this embodiment, the pressure monitored at the sensor 44 is relayed via the transmitter 48 to a user. The user will record the pressure measurement during rotation and reciprocation of the drill pipe. When the lower seal 20 then begins to fail through wear of the inner surface 32, he will note pressure measurements which gradually increase indicating that the lower stripper rubber or seal 20 is about to fail. On this indication, the user will halt the drilling operation so that the spindle 12 can be removed and the seals 20, 24 inspected and replaced. As the seal is replaced before it fails, the well fluid is prevented from ever travelling through the head 10 and exiting into the environment at the upper end 58. This is a safety feature of the diverter head 10 of the present invention.

Prior to removing the spindle 12, the quick release coupling 42 can be uncoupled to locate a hose onto the valve 40. By opening the valve 40, which preferably includes a needle valve, pressure can be bled from the portion 50 to allow for the safe removal of the spindle 12. This bleeding off of residual pressure between the seals 20, 24 can be made after any pressure measurement has been taken.

For the embodiment were the pressure sensor 44 is located on the head 10 only when a measurement is required a record of pressure measurements can be kept, for example, at the end of each 12 hour shift. It will then be possible to identify wear on the lower seal 20, which would manifest itself in the form of a gradual rise in differential pressure over a period of time.

An additional feature of the diverter head 10 is that of the coupling between the inlet flange 14 and the bowl 11. This coupling is as disclosed in WO 03/104609 and incorporated herein by reference.

The bowl 11 and flange 14 provide a bore 22 on a central axis through the head 10. The side outlet 16 is arranged to direct fluid in a perpendicular direction from the central bore 22. The bowl 11 and the flange 14 mate between a respective inner surface 60 of the bowl 11 and an outer surface 62 of the flange 14. The inner surface 60 includes a threaded bore 64 and a sealing bore 66. The diameter of sealing bore 66 is less than the diameter of threaded bore 64. The outer surface 62 of flange 14 includes a threaded section 68 and a sealing section 70. The threads of threaded section 68 engage the threads of threaded bore 64 of bowl 11. The sealing section 70 comprises a seal groove 72 into which is located an o-ring or rubber strip (not shown). When the threads of the threaded bore 64 engage the threads on the threaded section 68, the sealing section 70 locates against the sealing bore 66, thus providing sealing engagement between the bowl 11 and the flange 14 to prevent the egress of fluid from the head 10 at this location. The seal will be maintained as the threads are moved relative to each other so that the bowl 11 can rotate on the central bore 22 relative to the flange 14. This rotation is selective and continuous through 360 degrees around a central axis of the head 10.

Located around the flange 14 is a locking ring 74. Ring 74 is a threaded lock ring which comprises a threaded inner surface 76 that engages threaded section 78 of flange 14. Ring 74 can be rotated upwards towards the base 78 of bowl 11 to prevent movement of the bowl 11 and thus lock the bowl 11 to the flange 14.

To operate, lock ring 74 is threaded onto threaded section 48 of flange 14. Flange 14 is threaded into bowl 11 until face 86 of flange 14 contacts face 88 of bowl 11. Lock ring 74 is threaded until it contacts the base 78 of bowl 11. The rotating diverter head 10 is mounted onto annular blow out preventer of a stack (not shown) using lugs to assist its movement. Head 10 is fixed to blow out preventer 82 mating flange 14 to the outlet flange connection of the annular blow out preventer using threaded studs located through ports.

A return flow line is attached to the outlet flange 92 of the side outlet 16. The flow line is typically a section of fixed piping connected to a separator (not shown). If the flow line is not aligned with outlet flange 92, the bowl 11 can be rotated about the central axis until the axis of outlet flange 92 is co-linear with the axis of flow line.

Bowl 11 is rotated about its vertical axis by unthreading lock ring 74, rotating the bowl 11 on the threads of the threaded bore 64 against the threads of threaded section 68 of flange 14, until the axis of outlet flange 92 is co-linear with the axis of flow line. Lock ring 74 is then threaded upward and tightened against the base 58 of the bowl 11.

Reference is now made to FIG. 2 of the drawings, which illustrates a diverter head including a pressure monitoring system located upon a blow out preventer stack and a housing support according to a further embodiment of the present invention.

At the centre of FIG. 2, there is shown a well head in the form of a blow out preventer stack 100 upon which is located a rotating diverter head 10 as described herein, with reference to FIG. 1, and given the same reference numerals as those of FIG. 1 for clarity. The rotating diverter head 10 is located on the blow out preventer 100 via the flange 14 and the bowl 11 can be rotated to align the side outlet 16 with a return fluid line 102. A drill pipe 104 is located through the head 10 and rotated and reciprocated during use.

During rotation and reciprocation of the drill pipe 104, pressure measurements are recorded by the pressure sensor 44 via the port 36 on the head 10. The pressure measurements are transmitted by a radio transmitter 48 to a receiver 106 within a control unit 108, located on the top side of the well. An antenna 110 is used to improve the transmission and the pressure monitored is indicated on a digital panel 112 of the control unit 108. Micro-processors 114 within the control unit record each pressure measurement and determine the differential pressure between the consecutive measurements and display the pressure increase on the digital panel 112. When the pressure increase rises above a value which has been pre-set for the head 10, the micro-processor 114 will send a signal to a light 116 and a loud speaker 118 to provide an audible and visual signal that the seal 20,24 within the head 10 is nearing failure. The pre-set value will be determined by the nature of the seal within the head 10 and the well pressure expected within the well at the blow out preventer stack 100. Thus, an early warning system is provided to determine when the seal within the head 10 is about to fail. At this time, having been alerted by the audible and visual signals 116, 118, the operator will stop the drill pipe 104 and remove it from the well. At that time the blow out preventer stack 100 will seal off the well so that the diverter head 10 can be moved.

In the embodiment shown, only the housing or spindle 12 needs to be detached from the head 10, so that the bowl 11, inlet flange 14 and side outlet 16 can remain attached to the blow out preventer stack. This means that the spindle 12, together with the seals 20,24, can be lifted via lugs 114 over to a housing support 116 located beside the well head. The spindle 12 is located into a holder 118. Holder 118 is rotatably mounted 120 onto the frame 116. When supported, the seals 20, 24 located upon the spindle 12 can be replaced. As the spindle 12 can rotate via the mounts 120 this allows for easy removal and replacement of the seals 20, 24. As the seals are identical, a common batch of seals can be used and preferably these seals are of easy fit type for speed of replacement. The spindle 12 can be located into the bowl 11 again and the drill pipe 104 returned, and drilling can continue. By having the support at the well head, the stripper rubbers can be replaced quickly and easily to reduce the downtime.

The principal advantage of the present invention is that it provides an early warning of imminent failure of a seal such as a stripper rubber within a diverter head. This pre-failure warning allows for replacement of the stripper rubber prior to failure and thus reduces the safety risk at the well.

A further advantage of the present invention is that by recording pressure at the stripper rubber, the maximum pressure before failure can be set, so that the maximum life of the stripper rubber is obtained prior to replacement.

A yet further advantage of an embodiment of the present invention is that it provides a housing support so that the spindle only needs to be removed for stripper rubber replacement, and this can be done at the well head to reduce downtime.

Various modifications may be made to the invention herein described without departing from the scope thereof. For example, although FIG. 1 illustrates a dual stripper rubber system, the invention may be used with a single stripper rubber, where the pressure would be monitored behind the stripper rubber. Additionally any type of pressure sensor may be used. 

1. A rotating diverter head comprising: a bowl member having a first bore aligned on a central axis therethrough and a second bore located substantially transverse of the central axis; a housing located substantially within the bowl member including rotational means to rotate the housing relative to the bowl member; first sealing means to sealably engage upon a drill pipe when the drill pipe is inserted through the first bore, and pressure detecting means to determine pressure at a side of the first sealing means.
 2. A rotating diverter head as claimed in claim 1 wherein the first sealing means is a stripper rubber located on the housing such that the sealing means will rotate with the housing relative to the bowl member.
 3. A rotating diverter head as claimed in claim 1 wherein the head comprises a second sealing means to sealably engage upon the drill pipe, and wherein the second sealing means is located above the first sealing means so that the drill pipe engages the second sealing means prior to the first sealing means when inserted into the first bore.
 4. A rotating diverter head as claimed in claim 3 wherein the first and second sealing means are substantially the same stripper rubbers.
 5. A rotating diverter head as claimed in claim 3 wherein the pressure detecting means is located between the first and second sealing means.
 6. A rotating diverter head as claimed in claim 1 wherein the detecting means comprises a pressure sensor.
 7. A rotating diverter head as claimed in claim 6 wherein the pressure sensor is located at a port on the housing above the first sealing means.
 8. A rotating diverter head as claimed in claim 7 wherein the port is located between the sealing means, adjacent the second sealing means.
 9. A rotating diverter head as claimed in claim 7 wherein a valve is located in the port.
 10. A rotating diverter head as claimed in claim 7 wherein a quick release coupling is located at the port.
 11. A rotating diverter head as claimed in claim 6 wherein the pressure sensor includes a transmitter.
 12. A rotating diverter head as claimed in claim 1 wherein the detecting means comprises a warning means being an audible and/or visual indicator to alert a user when the pressure detected has reached a predetermined level indicating that the first sealing means should be changed.
 13. A rotating diverter head as claimed in claim 1 wherein the diverter head is supplied with a housing support, comprising a stand and a rotatably mounted holder adapted to support the housing when inserted therein.
 14. A method of preventing stripper rubber failure in a rotating diverter head, the method comprising the steps: (a) providing a port in the head above the stripper rubber; (b) connecting a pressure sensor to the port; (c) measuring the pressure at the port; (d) recording consecutive pressure measurements over time intervals; (e) noting when the measurements show an increase in pressure greater than a predetermined pressure value; and (f) replacing the stripper rubber.
 15. A method as claimed in claim 14 wherein the pressure value is chosen to be less than an operating pressure in the well.
 16. A method as claimed in claim 14 wherein the head includes dual stripper rubbers, and the pressure differential between the stripper rubbers is measured
 17. A method as claimed in claim 14 wherein the method further includes the step of bleeding off residual pressure once the pressure measurement has been taken.
 18. A method as claimed in claim 14 wherein the method further includes the step of providing an audible and/or visual indicator when the predetermined pressure value has been reached. This will alert a user to change the stripper rubber.
 19. A method as claimed in claim 14 wherein the method further includes the steps of: (a) removing a portion of the head including the stripper rubber; and (b) locating the portion on a rotatable support positioned adjacent a well head; before replacing the stripper rubber. 